Novel cyanine dyes for the sensitization of organic photoconductors



United States Patent 3,549,362 NOVEL CYANINE DYES FOR THE SENSITIZATION OF ORGANIC PHOTOCONDUCTORS James W. Carpenter, John D. Mee, and Donald W. Heseltine, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N .Y., a corporation of New Jersey No Drawing. Filed Apr. 12, 1968, Ser. No. 721,104

Int. Cl. G03g 5/10 US. Cl. 961.6 26 Claims ABSTRACT OF THE DISCLOSURE Organic photoconductors are spectrally sensitized with a polymethine dye featuring in an imidazole ring joined at the carbon atom in the 5-position of the imidazole ring to a dimethine linkage, the imidazole ring. having fused to the [a] side thereof the non-metallic atoms required to complete at least one fused ring, and a second desensitizing nucleus joined at a carbon thereof to the dimethine linkage.

This invention relates to electrophotography, and more particularly to materials and elements useful in the electrophotographic process.

Elements useful in the electrophotographic process commonly comprise an electrically conductive support bearing a stratum including a photoconductive insulating layer which has a resistivity substantially greater in the dark than in light actinic thereto. Such elements can be used in electrophotographic processes, for example, by first adapting the element in the dark to obtain a uniformly high resistivity in the photoconductive insulating layer, and electrostatically charging the element in the dark to obtain a relatively high potential which may be either negative or positive in polarity. The element can then be exposed to a light pattern which lowers the resistivity and thereby the charge density of the illuminated areas imagewise in proportion to the intensity of illumination incident upon each point of the illuminated areas. A latent electrostatic image is obtained. Visible images can be formed from the latent electrostatic image in any convenient manner, such as by dusting with a finely divided, fusible pigment the particles of which bear an electrostatic charge opposite that remaining on the surface of the photoconductive insulating layer. Thereafter, the pigment particles can be fused to the surface to provide a permanent image.

Various photoconductive substances have been em ployed in photographic elements and processes of the type described above. Typical inorganic photoconductive materials include selenium and zinc oxide. Such inorganic photoconductive materials have inherent disadvantages, such as an inability to be readily adapted to reflex copying systems, or to produce images on transparent supports except by indirect means. Organic photoconductors avoid such disadvantages, but, generally have relatively poor sensitivity to visible radiation. It has been proposed to increase the spectral sensitivity of organic photoconductors with certain cyanine or merocyanine dyes, for example, those listed in Table D hereinafter. The spectral sensitivity imparted by such dyes is very weak. It therefore appears highly desirable to provide effective spectral sensitizers for organic photoconductors.

One object of this invention is to provide novel sensitized organic photoconductors.

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Another object of this invention is to provide novel spectrally sensitized organic photoconductor materials.

Still another object of this invention is to provide novel compositions of matter comprising organic photoconductors and certain spectral sensitizers.

A further object of this invention is to provide novel compositions of matter comprising organic photoconductor, binder and certain spectral sensitizers for the organic photoconductor.

Still another object of this invention is to provide a novel electrophotographic material including a conductive support having coated thereon an insulating layer containing spectrally sensitized organic photoconductor.

A further object of this invention is to provide methods for spectrally sensitizing organic photoconductors.

Still other objects of this invention will be apparent from the following disclosure and the appended claims.

In accordance with one embodiment of this invention, novel compositions of matter are provided comprising organic photoconductors spectrally sensitized with the dyes defined more fully below. These compositions can be incorporated in a suitable binder and coated on a conductive support for use in electrophotography.

In another embodiment of this invention, compositions of matter are provided comprising organic photoconductors spectrally sensitized with the dyes described below, dispersed in an insulating binder. These compositions of matter can be coated on a conductive support and used in electrophotographic processes.

In still another embodiment of this invention, electrophotographic materials are provided comprising a conductive support having coated thereon a layer comprising an insulating binder, an organic photoconductor and a spectral sensitizing quantity of a dye defined more fully below.

In another embodiment of this invention, a method is provided for spectrally sensitizing organic photoconductors which comprises mixing a dye of the type described below with an organic photoconductor, in a concentration suflicient to effectively spectrally sensitize the organic photoconductor. Preferably, the dye and organic photoconductor are mixed in a suitable solvent.

The spectral sensitizing dyes which are employed in this invention are certain polymethine dyes containing certain fused imidazole nuclei which, when incorporated in a test negative gelatin silver b-romoiodide emulsion consisting of 99.35 mole percent bromide and .65 mole percent iodide, at a concentration of 0.2 millimole of dye per mole of silver halide, desensitize the emulsion more than 0,4 log B when the test emulsion is coated on a support, exposed through a step wedge in a sensitometer (to obtain D to light having a wavelength of 365 nm., processed for three minutes at 20 C. in Kodak Developer D-19, and is fixed, washed and dried. As used herein and in the appended claims, the test negative silver bromoiodide emulsions are prepared as follows:

In a container with temperature control is put a solution with the following composition:

Potassium bromide: g.- Potassium iodide: 5 g. Gelatin: 65 g.

Water: 1700 cc.

and in another container is put a filtered solution consisting of:

Silver nitrate: 200 g. Water: 2000 cc.

v G. N-methyl-p-aminophenol sulfate 2.0 Sodium sulfite, dessicated 90.0 Hydroquinone 8.0 Sodium carbonate, monohydrated 52.5 Potassium bromide 5.0

Water to make 1.0 liter.

As indicated above, the polymethine dyes employed in this invention desenitize conventional negative silver halide emulsions. Such emulsions are inherently sensitive to blue radiation. The present dyes reduce that sensitivity. In addition, these dyes fail to provide practical spectral sensitization for such emulsions. Therefore, it was quite unexpected to find that they spectrally sensitized organic photoconductors.

Another characteristic of the dyes of this invention is that they are substantially non-photoconductive. The term substantially non-photoconductive as used herein means that no image is formed when a solution of 0.002 g. of the dye and 0.5 g. of polyester binder (described in Examples l to 14 below) are dissolved in 5.0 ml. of methylene chloride, and is coated and tested (in the absence of any photoconductor) as described in Examples 1 to 14 below.

The dyes of this invention increase the speed of organic photoconductors by extending or increasing the response of the photoconductor to visible radiation (i.e., radiation in the range of about 400 nm. to 700 nm.). In the concentrations used, the dyes herein appear to function as spectral sensitizers when employed with efficient organic photoconductors. When the organic photoconductor used is poor or inefficient, the dyes seem to function as speed increasing compounds as well as spectral sensitizers.

The polymethine dyes that are useful in practicing the invention include those comprising first and second 5- to G-membered nitrogen containing heterocyclic nuclei joined together by a dimethine linkage; said first nucleus comprising an imidazole ring joined at the carbon atom in the 5-position (which is not necessarily the 5-position in the numbering of the compound) to said linkage, said imidazole ring having fused, on the [a] side thereof, the non-metallic atoms necessary to complete at least one ring containing from 5 to 6 atoms; and, said second nucleus being an electron-accepting nucleus of the type used in'cyanine dyes joined at a carbon atom thereof to said linkage. Reference should be made to Formulas I and II below for the numbering used for the imidazole ring. These numbers are generally not retained in numbering specific compounds, as will be apparent from the examples.

Some highly useful dyes of the invention include those comprising first and second 5- to 6-membered nitrogen containing heterocyclic nuclei joined by a dimethine linkage; the first of said nuclei comprising an imidazole ring joined at the 5 carbon atom thereof to said linkage, said imidazole ring having fused on the [a] side thereof the atoms necessary to complete a nucleus selected from the group consisting of an imidazole-[1,2-a] pyridine nucleus, an imidazo[2,1-b]thiazole nucleus, an imidazo[2,1-b] oxazole nucleus, an imidazo[2,1-b]selenazole nucleus, an imidazo[2,l-b]-l,3,4-thiadiazole nucleus; and said second nucleus being an electron-accepting nucleus of the type used in cyanine dyes joined at a carbon atom thereof to said linkage.

The preferred polymethine dyes that are useful herein include those represented by the following general formulas:

wherein n represents a positive integer of from 1 to 2, L represents a methine linkage, e.g.,

etc., R represents a hydrogen atom, an alkyl group such as methyl, ethyl, propyl, isopropyl, butyl, hexyl, cyclohexyl, decyl, dodecyl, etc., or preferably an aryl group, e.g., phenyl, tolyl, xylyl, bromophenyl, chlorophenyl, nitrophenyl, methoxyphenyl, naphthyl, phenlazophenyl, etc; R and R each represents an alkyl group (including substituted alkyl), preferably a lower alkyl group containing from 1 to 4 carbon atoms, e.g., methyl, ethyl, propyl, isopropyl, butyl, hexyl, cyclohexyl, decyl, dodecyl, etc., and alkoxyalkyl group, e.g., B-methoxyethyl, w-butoxybutyl, etc., a hydroxyalkyl group, e.g., fi-hydroxyethyl, whydroxybutyl, etc., a carboxyalkyl group, e.g., fl-carboxyethyl, w-carboxybutyl, etc., a sulfoalkyl group, e.g., flsulfoethyl, -sulfobutyl, w-sulfobutyl, etc., a sulfatoalkyl group, e.g., p-sulfatoethyl, w-sulfobutyl, etc., an acyloxyalkyl group, e.g., fi-acetoxyethyl, 'y-propionyloxypropyl, w-butyryloxybutyl, etc., an alkoxycarbonylalkyl, e.g., flmethoxycarbonylethyl, w-methoxycarbonylbutyl, etc. and the like, or an alkeny group, e.g., allyl, l-propenyl, Z-butenyl, etc., or an aryl group, e.g., phenyl, tolyl, xylyl, chlorophenyl, methoxyphenyl, naphthyl, etc., and the like; X6 represents an acidanion, e.g., chloride, bromide, iodide, thiocyanate, sulfamate, perchlorate, ptoluenesulfonate, methyl sulfate, ethyl sulfate, etc.; Z represents the non-metallic atoms necessary to complete an electron-accepting hetercyclic nucleus selected from the group including a nitrobenzothiazole nucleus, e.g., 5-

'nitrobenzothiazole, 6-nitrobenzothiazole, 5-chloro-6-nitrobenzothiazole, etc.; a nitrobenzoxazole nucleus, e.g., 5- nitrobenzoxazole, -6-nitrobenzoxazolle, 5-chloro-6-nitrobenzoxazole, etc.; a nitrobenzoselenazole nucleus, e.g., 5- nitrobenzoselenazole, 6-nitrobenzoselenazole, 5-chloro-6- nitrobenzoselenazole, etc.; an imidazo [4,5-b]-qu inoxaline nucleus, e.g., imidazo [4,5-b]quinoxaline, 1,3-dialkylimidazo[4,5-b]quinoxaline such as 1,3-diethylimidazo- [4,5-b]quinoxaline, 6 chloro 1,3-diethylimidazo[4,5-b] quinoxaline, etc., 1,3-dialkenylimidazo [4,5-b] quinoxaline such as 1,3-diallylimidazo [4,5-b] quinoxaline, 6-chloro-l,3- diallylimidazo[4,5-b]quinoxaline, etc., 1,3-diarylimidazo [4,5-b1quinoxaline, such as 1,3-diphenylimidazo[4,S-b] quinoxaline, 6 chloro 1,3-diphenylimidazo[4,5-b]quinoxaline, etc.; a 3,3-dialkyl-3H-pyrrolo[2,3b]pyridine nucleus, e.g., 3,3-dimethyl1-3H-pyrrolo[2,3-b]pyridine 3,3-diethyl-3H-pyrrolo[2,3-b1pyridine, etc.; a 3,3-dialkyl-3H- nitroindole, e.g., 3,3 dimethyl 5-nitro-3H-indole, 3,3- diethyl-S-m'tro-SH-indole, 3,3 dimethyl 6 nitro 3H indole, etc.; a thiazole [4,5-b]quinoline nucleus; or a nitroquinoline, e.g., S-nitroquinoline, 6-nitroquinoline, etc.; and Q represents the non-metallic atoms required to complete at least one fused heterocyclic ring containing from 5 to 6 atoms in said ring, which ring may also contain a second or third hetero atom such as oxygen, sulfur, selenium or nitrogen such as a pyridine ring, e.g., pyridine, an alkylpyridine such as a methylpyridine, and ethylpyridine, etc., a chloropyridine, a methoxypyridine, a nitropyridine, a phenylpyridine, etc. ring; a thiazole ring, e.g., thiazole, 4-methylthiazole, S-methylthiazole, 4,5-dimethylthiazole, 4-phenylthiazole, S-phenylthiazole, 4,5-diphenylthiazole, etc. ring; an oxazole ring, e.g., oxazole, 4-methyloxazole, S-methyloxazole, 4,5-dimethyloxazole, 4-phenyloxazole, S-phenyloxazole, etc. ring; a selenazole ring, e.g., selenazole, 4-methylselenazole, S-methylselenazole, 4,5- dimethylselenazole, 4 phenylselenazole, 5 phenylselenazole, etc. ring; a 1,3,4-thiadiazole ring, e.g., 1,3,4-thiadiazole, a 2-alkyl-1,3,4-thiadiazole such as 2-methyl-1,3,4- thiadiazole, etc., a 2-aryl-l,3,4-thiadiaz0le such as 2-phenyl-1,3,4-thiadiazole, etc. 2-chloro-1,3,4-thiadiazole, 2-nitro- 1,3,4-thiadiazole, etc. ring; a pyridazine, e.g., pyridazine, an alkylpyridazine such as 3-methylpyridazine, 3-butylpyridazine, etc., a chloropyridazine such as 3-chloropyridazine, etc., an arylpyridazine such as 3-phenylpyridazine, 3,6-diphenylpyridazine, etc., an alkoxypyridazine such as 4-ethoxypyridazine, a quinoline ring, etc. Other desensitizin nuclei defined by Z in above Formula I that are useful include nitronaphthothiazole, nitronaphthoxazole, nitroselenazole, nitronaphthoselenazole, and nitropyridine, and the like. The monosalt dyes defined by Formula I above provide particularly eflicacious spectral sensitizing dyes for the photoconductor compositions and elements of this invention.

As used herein and in the appended claims, electron accepting nucleus refers to those nuclei which, when converted to a symmetrical carbocyanine dye and added to a gelatin silver chlorobromide emulsion containing 40 mole percent chloride and 60 mole percent bromide, at a concentration of from 0.01 to 0.2 grams dye per mole of silver, caused by electron trapping at least about an 80 percent loss in the blue speed of the emulsion when sensitometrically exposed and developed three minutes in Kodak developer D-l9 at 20 C., the composition of which is given above. Preferably, the electron-accepting nuclei are those which, when converted to a symmetrical carbocyanine dye and tested as just described above, essentially completely desensitize the test emulsion to blue radiation. Substantially complete desensitization as used herein, results in at least a 90 percent, and preferably a 95 percent loss of speed to blue radiation.

The polymethine dyes defined by Formula I above are conveniently prepared, for example, by heating a mixture of (1) a heterocyclic compound of the formula:

III Z wherein n, R R X9 and Z are as previously defined, and R represents methyl, ethyl, benzyl, etc., and (2) a heterocyclic compound of the formula:

wherein R and Q are as previously defined, in approximately equimolar proportions, in a suitable solvent medium such as acetic anhydride. The crude dyes are then separated from the reaction mixtures and purified by one or more recrystallizations from appropriate solvents such as ethanol, methanol, cresol/methanol mixtures, etc.

To prepare the dyes defined by Formula II above, a mixture of (1) a compound of above Formula III and (2) a compound of the formula:

wherein R, R and Q are as previously defined, is reacted and the purified dyes obtained in a manner similar to that described above for preparing the dyes of Formula I above.

The intermediates defined by Formula -IV above are conveniently prepared by formulating a compound of the formula:

wherein R and Q are as previously defined, by the Vilsrneier reaction, comprising treatment of Compound VI with a complex of phosphoryl chloride and dimethylformamide in excess dimethylformamide as solvent, followed by hydrolysis with aqueous sodium hydroxide. After chilling, the precipitated crystals are collected, and washed well with water, and further purified, when necessary, by one or more recrystallizations from solvents such as acetone, ethanol, chloroform/ ethanol mixtures, and the like. The salt intermediates defined by Formula V above are readily prepared by quaternizing compounds represented by Formula IV above with any suitable quaternizing reagents, such as represented by methyl iodide, dimethyl sulfate, diethyl sulfate, methyl p-toluenesulfonate, and the like. Further details for the preparation of the dyes herein can be had by reference to our copending application Ser. No. 677,058 filed Oct. 23, 1967, wherein such dyes and their preparations are described.

Included among the dyes of Formula I above are the following typical dye compounds. The method for pre- 7 paring Dye No. I is included in Table A below to illustrate, in general, how the dyes herein are prepared.

TABLE A Compound Dye No;

I 3-ethyl-6-nitro.2-[2- (Z-phenylim idazo[1,2-a]pyrid-3-y1) vinyl]benzothiazolium iodide.

See footnote at end of table II 1,3-diphenyl-2-[2-(2-phenylimidazo[1,2-a]pyrid3-yl)-vinyl] imidazo[4,5-b]quinoxalinium iodide.

III- 3-ethyl-2-[2-(1-methyl-Z-phenyl-lH-imidazo[1,2-a1-pyridinium-3-yl) vinyl]-6-n1trobenzothiazolium di-iodide.

V 3-ethyl-2-I2-[S-methyl-2-(4-niti'ophenyl)imidazo[1,2-a]- pyrid-3-yl1vinyl)-6-nitrobenzothiazolium iodide.

VI 1,3diethyl-2-{2-[8-methyl-2-(4-nitrophenyl)imidazo[1,2-

a]pyrid3-y1]vinyl}imidazo[4,5-b]quinoxalinium ptoluenesulfonate.

IX 1,3,3-trimethyl-2-{ 2-[8-methyl-2-(i-phenylazophenyl) imidazo[1,2-a1pyrid-3-yl1viuyl]-5nitro-3H-iudolium ptoluenesulionate.

X 3-ethyl-2-l2-[8-methyl-2-(4-phenylazophenyl)imidazo-[1,2-

awfyriditl-yflvinyll-6-nitrobenzothiaozlium p-toluenesu ona e.

XI 1,3-dieth yl-2-{2-[8-methyl-2- (4-ph enylazoph enyl) imidazo [1-2-a1pyrid-3-yl1viuyl]imidazo[4,5-b]quinoxalinium ptoluenesulfonate.

XIII.-- 6-chloro-2-l2-[7-methyl-2-(4-nitrophenyl)imidazo[1,2-b]- pyn'd-3-yl]vinyl}-1,3-diphenylimidazo[4,5-b]quinoxalinium iodide.

XIV. 1,2,3trimethyl-2-[ 2-[7-methyl-2- (4-nitroph enyl) imidazo- [lfi-ralpyriddyllvinyll-5-nitro-3H-indolium p-toluencs onate.

XV... 1,3,3-trlmethyl-5-nitro-2-{ 2-[6-(4-nitrophenyl)imidazo- [2,1-b]thiazol-5-yl]vinyl}-3H-indolium p-toluenesulfonate.

CH3 CH s CH==CH- N E OSO2C H yl]vinyl}imidazo[4,5 b quinoxalinium!p-toluenesulionate.

XVII..- (rchloroQ-I2-[6-(4-nitrophenylimidazo[2,l-b]thiazol-5-yl] vinyll 1,3-diphenylimidazo[4,5-blquinoxalinium ptoluenesulionate.

TABLE AContinued Compound Dye FNo.

XVIIL. 1,3,3-trirnethyl-2-{2-[2-methyl-6-(4-nitrophenyl) imidazo [2,l-b]1,3,4-thiadiazol-5yl]vinyll-fi-nitro-3H-indolium ptoluenesulfonate.

CH3 CH XIX 3-ethyl-2-{ Z-[Z-methyl-G-(4-nitrophenyl) imidazo[2, 1-b]- 1,3,4-thiadiazol-5yl]vinyl i-fi-nitrobenzothiazolium ptoluenesulfonate.

XX. 1,3-diethyl-2-{ 2-[2-methyl-6- (-nitrophenyl) imidazo- [2, l-b]1,3,4-thiadiazol-5-yl]vinyllimidazo[4,5-b]- quinoxalinium p-toluenesulfonate.

XXL 2-{ 2-[2-(4-bromophenyl)-6-chloroimidazo[1,2-b]pyridazin- 3-yl]vinyl}-3-ethyl-6-nitrobenzothiazolium ptoluenesulfonate.

XXII. 2-[ 2-[2- (4-brornophenyl-6-chloroimidazo[1,2-b1pyridazin- 3-yl]vinyll-l,3-diethylimidazo[4,5-b]quinoxalinium ptoluenesulionate.

XXIII- 2-{2-[2-(tbrornophenyl-fi-methoxyimidazoll,2-b]pyridazin-3-yl1vinyl}-1,3,3-trimethyl-5-uitro-3H-indolium p-toluenesulfonate.

XXIV 2-{2-[2-(4-br0mophenyl)-6-ehloroimidazo[1,2-b1pyridazin- 3-yl]vinyl l-1,3,3-trimethyl-5-nitro3H-indolium ptoluenesulfonate.

XXV..- 2-{ 2-[2-(4-brornophenyl) -6-methoxyimidazo[1,2-b]-pyridazin-3-yl1vinyll-3-ethyl-6-nitrobenzothiazolium ptoluenesulionate.

XXVL- 2-{2-[2-(4-bromophenyl) -6-methoxyimidazo[l,2-b]- pyridazin-3-yl1vinyl}-l,3-diethylirnidazo[4,5-b1- quinoxalinium p-toluenesulionate.

N o'rE.3-Formyl-2-phenylimidazo[1,2-a]pyridine (1.11 g., 1 mol.) and 3-ethyl-2-methyl-6-nitrobenzothiazolium p'toluene sulfonate (1.98 g., 1 mol.) in acetic anhydride (10 ml.) are heated at reflux for 5 minutes. The cooled mixture is diluted with ether. The ether layer is decanted and the sticky residue dissolved in hot methanol (25 1111.). A solution of sodium iodide (2.0 g) in a little water is added, the mixture chilled, and the solid collected. After two recrystallizations from methanol, the yield of purified dye is 0.60 g. (21%), m.p. greater than 179 C.

It will be apparent from the foregoing that other dyes related to above Dyes No. I to XIV having generally similar spectral sensitizing properties for the photoconductor compositions and elements of this invention can be prepared, in general, by the processes described above including, for example, the dye l-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-6-nitro-2-[2-(2- phenylimidazo[1,2-a]pyrid 3 yl)-vinyl]benzoxazolium salt (e.g., the chloride, bromide, iodide, perchlorate, ptoluenesulfonate, etc. salt); the dye l-alkyl- (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-6-nitro-2-[2- (2-phenylimidazo[1,2-a]pyrid 3 yl)vinyl] selenazolium salt (e.g., the chloride, bromide, iodide, perchlorate, ptoluenesulfonate, etc. salt); the dye 1,3-dialkyl (e.g., wherein alkyl is methyl; ethyl propyl, isopropyl, butyl, decyl, etc)-2{2-[7-(or 8-) methyl-2- (4-nitrophenyl)imidazo 1 ,2-a] -pyrid-3-yl] vinyl}imidazo [4,5 b] quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate,

p-toluenesulfonate, etc. salt); the dye l,3-diallyl-2-{2-[8 methyl-2-(4-nitrophenyl)imidazo[1,2 a] pyrid 3 yl] vinyl}imidazo[4,5-b] quinoxalim'um salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); the dye 1,3-dialkyl (e.g., wherein alkyl is methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-6-chloro-2-{2- [7- (or 8-) methyl-Z- (4-nitrophenyl) imidazo 1 ,2-a] -pyrid- 3-yl]vinyl}-imidazo[4,5-b]quinoxalinium salt, (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt), and the like. Other similarly useful dyes herein that are related to Dyes No. XV to XVII include, for example, the dye 3-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-2-{2-[6-(4-nitrophenyl)imidazo [2,1-b] thiazol-S-yl] -vinyl}-6 nitrobenzothiazolium salt (e.g., the chloride, bromide, iodide, perchlorate, ptoluenesulfonate, etc. salt; the dye 3-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-2-{2-[6-(4- nitrophenyl)imidazo[2,1-b]thiazol--yl1vinyl} 6 nitrobenzoxazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); the dye 3-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)- 2-{2-[6-(4-nitrophenyl)imidazo[2,1 b]selenazol 5-yl] vinyl}-6-nitrobenzoselenazolium salt (e.g., chloride, ptoluenesulfonate, etc. salt); the dye 2-[2-(6-phenylirnidazo [2,1-b]thiazol-5 yl)vinyl]-1,3-diphenylimidazo[4,5- bJquinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); and the like dyes. Other similarly useful dyes herein that are related to Dyes No. XVIII to XX include, for example, the dye 3-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-2-{2-[2 methyl 6 (4-nitrophenyl)imidazo[2,1- b]1,3,4-thiadiazol-5-yl]vinyl} 6 nitrobenzothiazolium salt (e.g., the chloride, bromide, iodide, perchlorate, ptoluenesulfonate, etc. salt); the dye 3-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-2-{2- [2-methyl- 6-(4-nitrophenyl)-imidazo[2,1 b] 1,3,4 thiadiazol-S- yl]vinyl}-6-nitrobenzoxazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); the dye 2-[2-(6-phenylimidazo-[2,l-b]-l,3,4-thiadiazol-5-yl)vinyl]-1,3-diphenylimidazo [4,5-b] quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); and the like dyes. Still other similarly useful dyes herein that are related to Dyes No. XXI to XXVI include, for example, the dye 2-{2-[2-(4-chlorophenyl)-6-chloroimidazo[1-2 b)pyridazin-3-yl]vinyl}-3-alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)-6-nitrobenzothiazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); the dye 2-{2-[2-(4-bromophenyl)-6- methoxyimidazo[1,2-b]pyridazin-3-yl]-vinyl} 3 alkyl (e.g., methyl, ethyl, propyl, isopropyl, butyl, decyl, etc.)- 6-nitrobenzoxazoliun1 salt (e.g., the chloride, bromide, iodide perchlorate, p-toluenesulfonate, etc. salt); the dye 2-{2-[2-(4=bromophenyl)-6-chloroimidazo[1,2 b]pyridazin-3-yl]vinyl}-1,3 diphenylimidazo[4,5 b]quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt); and the like dyes.

Dyes such as illustrated above can be used alone, or a combination of one or more of the above described dyes can be used to impart the desired spectral sensitivity. All of them are spectral sensitizers for organic photoconductors. Suitable organic photoconductors which are effectively spectrally sensitized by such dyes include both monomeric and polymeric organic photoconductors. The invention is particularly useful in increasing the speed of organic photoconductors which are substantially insensitive, or which have low sensitivity (e.g., a speed less than 25 but generally less than 10 when tested as described in Examples 1 to 14 below) to radiation of 400 to 700 nm.

An especially useful class of organic photoconductors is referred to herein as organic amine photoconductors. Such organic photoconductors have as a common structural feature at least one amino group. Useful organic photoconductors which can be spectrally sensitized in accordance with this invention include, therefore, arylamine compounds comprising 1) diarylamines such as diphenylamine, dinaphthylamine, N,N'-diphenylbenzidene, N- phenyl-l-naphthylamine; N-phenyl 2 naphthylamine; N,N'-diphenyl-p-phenylenediamine; 2 carboxy-S-chloro- 4-methoxydiphenylamine; p-anilinophenol; N,N-di-2- napthyl-p-phenylene diamine; 4,4-benzylidene-bis(N,N- diethyl-m-toluidine), those described in Fox US. Pat. 3,240,597 issued Mar. 15, 1966, and the like, and (2) triarylamines including (a) nonpolymeric triarylamines, such as triphenylamine, N,N,N',N'-tetraphenyl-m-phenylenediamine; 4-acetyltriphenylamine, 4-hexanoyltriphenylamine; 4-lauroyltriphenylamine; 4-hexyltriphenylamine, 4-do-decyltriphenylamine, 4,4 bis(diphenylamino)benzil, 4,4-bis(diphenylamino)-benzophenone, and the like, and (b) polymeric triarylamines such as poly[N,4"- (N,N,N triphenylbenzidine)]; polyadipyltriphenylamine, polysebacyltriphenylamine; polydecarnethylenetriphenylamine; poly N (4-vinylphenyl)-diphenylamine, poly-N-(vinylphenyl)-a,a-dinaphthylamine and the like. Other useful amine-type photoconductors are disclosed in US. Pat. 3,180,730, issued Apr. 27, 1965.

Other very useful photoconductive substances capable of being spectrally sensitized in accordance with this invention are disclosed in Fox US. Pat. 3,265,496 issued Aug. 9, 1966, and include-those represented by the following general formula:

wherein A represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear, (e.g., phenylene, naphthylene, biphenylene, binapththylene, etc.), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy, etc.), or a nitro group; A represents a mononuclear or polynuclear monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.); or a substituted monovalent aromatic radical wherein said subsitituent can comprise a member, such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogen atom, a halogen atom or an aromatic amino group, such as A'NH; b represents an integer from 1 to about 12, and G represents a hydrogen atom, a mononuclear or polynuclear aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), a substituted aromatic radical wherein said substituent comprises an alkyl group, an alkoxy group, an acyl group, or a nitro group, or a poly (4-vinylphenyl) group which is bonded to the nitrogen atom by a carbon atom of the phenyl group. Certain nitrogen heterocyclic compounds are also useful photoconductors in the invention such as, for example, 1,3,5 triphenyl 2 pyrazoline, 2,3,4,5- tetraphenyl-pyrrole, etc.

Polyarylalkane photoconductors are particularly useful in producing the present invention. Such photoconductors are described in US. Pat. 3,274,000; French Pat. 1,383,461 and in a copending application of Seus et al. Ser. No. 624,233, Photoconductive Elements Containing Organic Photoconductors filed Mar. 20, 1967. These photoconducto. include leucobases of diaryl or triaryl methane dye salts, 1,1,1-triarylalkanes wherein the alkane moiety has at least two carbon atoms and tetraarylmethanes, there being substituted an amine group on at least one of the aryl groups attached to the alkane and methane moieties of the latter two classes of photoconductors which are non-leuco base materials.

Preferred polyaryl alkane photoconductors can be represented :by the formula:

I-(IlE G wherein each of D, E and G is an aryl group and J is a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and G containing an amino substituent. The aryl groups attached to the central carbon atom are preferably phenyl groups, although naphthyl groups can also be used. Such aryl groups can contain such substituents as alkyl and alkoxytypically having 1 to 8 carbon atoms, hydroxy, halogen, etc. in the ortho, meta or para positions, ortho-substituted phenyl being preferred. The aryl groups can also be joined together or cyclized to form a fluorene moiety, for example. The amino substituent can be represented by the formula \R4 wherein each R.,, can be an alkyl group typically having 1 to 8 carbon atoms, a hydrogen atom, an aryl group, or together the necessary atoms to form a heterocyclic amino group typically having 5 to 6 atoms in the ring such as morpholino, pyridyl, pyrryl, etc. At least one of D, E and G is preferably p-dialkylaminophenyl group. When I is an alkyl group, such an alkyl group more generally has 1 to 7 carbon atoms.

Representative useful polyarylalkane photoconductors include the compounds listed below:

TABLE B Compound No.:

(1) 4,4-bis- (diethylamino)-2,2'-dimethyltriphenyl-methane.

(2) 4'4"-diamino-4-dimethylamino-2',2"-

dimethyltriphenylmethane.

(3) 4,4-bis(diethylamino)-2,6-dichloro-2',2"- dimethyltriphenylmethane.

As described herein a wide variety of photoconductor compounds can be spectrally sensitized with the dyes referred to above. Some organic photoconductors will, of course, be preferred to others; but in general useful results may be obta'med from substantially all of the presently known organic photoconductors.

12 The following Table C comprises a partial listing of U.S. patents describing such organic photoconductor and compositions which can be used in place of those more particularly described herein. 1

TABLE C Patent Inventor Issued numbers Feb.25, 1964 3,122,435

. Mar. 31, 1964 Nov:24,1964 Dec. 15, 1964- Casslers Tomanek- Schlesinger.

Schlesinger- Schlesinger- Schlesinger Kosche et a1- Noe et a1 Sept. 20, 1966 The spectrally sensitized organic photoconductor compositions of this invention can, in certain arrangements, be employed in electrophotographic elements in the absence of binder. For example, the photoconductor itself is sometimes capable of film formation, and therefore requires no separate binder. An example of such film-forming photoconductor is poly(vinylcarbazole). However, the more common arrangement is to provide a binder for the spectrally sensitized organic photoconductive materials. Any suitable binder material can be utilized for the spectrally sensitized organic photoconductors of the invention. Such binders should possess high dielectric strength, and have good insulating properties (at least in the absence of actinic radiation) as well as good film forming properties. Preferred binder materials are polymers such as polystyrene, poly(-methylstyrene), styrenebutadiene polymers, poly(-vinyl chloride), poly- (vinylidene chloride), poly(vinyl acetate), vinyl acetatevinyl chloride polymers, poly(vinyl acetals), polyacrylic and methacrylic acid esters, polyesters such as poly(ethylene alkaryloxy-alkylene terephthalates), phenol-formaldehyde resins, polyamides, polycarbonates and the like.

The photoconductive compositions of the invention can be coated on any of the electrically conductive supports conventionally used in electrophotographic processes, such as metal plates or foils, metal foils laminated to paper or plastic films, electrically conductive papers and films, papers and films coated 'with transparent electrically conductive resins and the like. Other useful conducting layers include thin layers of nickel coated by high vacuum deposition and cuprous iodide layers as described in U.S. Pat. 3,245,833. Transparent, translucent or opaque support material can be used. Exposure by reflex requires that the support transmit light while no such requirement is necessary for exposures by projection. Similarly transparent supports are desired if the reproduction is to be used for projection purposes; translucent supports are preferred for reflex prints; and opaque supports are adequate if the image is subsequently transferred by any means to another support, the reproduction is satisfactory as obtained, or the reproduction is 13 to be used as a printing plate for preparing multiple copies of the original.

The quantity of the above-described dye required to spectrally sensitize an organic photoconductor varies with the results desired, the particular dye used, and the particular organic photoconductor used. Best results are obtained with about .01 to parts by weight dye and about 1 to 75 parts by weight of the organic photoconductor based on the photoconductive com position. Binder can be employed in such compositions, when desired, at preferred ranges of to 99 parts by weight. In addition, the composition can contain other sensitizers, either spectral sensitizers or speed increasing compounds, or both.

As used herein and in the appended claims, the terms insulating and electrically conductive have reference to materials the surface resistivities of which are greater than 10 ohms per square unit (eg. per square foot) and less than 10 ohms per square unit (e.g., per square foot) respectively.

Coating thicknesses of the photoconductive compositions of the invention on a support can vary widely. As a general guide, a dry coating in the range from about 1 to 200 microns is useful for the invention. The preferred range of dry coating thickness is in the range from about 3 to 50 microns.

To produce a reproduction of an image utilizing the electrophotographic elements of our invention, the photoconductive layer is preferably dark adapted, and then is charged either negatively or positively by means of, for example, a corona discharge device maintained at a potential of from 60007000 volts. The charged element is then exposed to light through a master, or by reflex in contact with a master, to obtain an electrostatic image corresponding to the master. This invisible image may then be rendered visible by being developed by contact with a developer including a carrier and toner. The carrier can be, for example, small glass or plastic balls, or iron powder. The toner can be, for example, a pigmented thermoplastic resin having a grain size of from about 1-100/L which may be fused to render the image permanent. Alternatively, the developer may contain a pigment or pigmented resin suspended in an insulating liquid which optionally may contain a resin in solution. If the polarity of the charge on the toner particles is opposite to that of the electrostatic latent image on the photoconductive element, a reproduction corresponding to the original is obtained. If, however, the polarity of the toner charge is the same as that of the electrostatic latent image, a reversal or negative of the original is obtained.

Although the development techniques described hereinabove produce a visible image directly on the electrophotographic element, it is also possible to transfer either the electrostatic latent image, or the developed image to a second support which may then be processed to obtain the final print. All of these development techniques are well known in the art and have been described in a number of US. and foreign patents.

The following examples are included for a further understanding of the invention.

EXAMPLES 1 TO 14 These examples show the great increase in speed of organic photoconductors when the dyes employed in this invention are added thereto. This increase in speed is due to the spectral sensitivity imparted to the photoconductor by the dyes described herein. The examples also show that the maximum sensitivity peaks (Abs max.) occur in most cases at wavelengths in the region of the spectrum from about 350 to 625 nm. A number of the dyes also impart more than one maximum sensitivity peak as indicated in Table l hereinafter.

A series of solutions are prepared consisting of 5.0 ml. methylene chloride (solvent); 0.15 g. 4,4-bis(diethylamino)-2,2'-dimethyltriphenylmethane (organic photoconductor); 0.50 g. polyester composed of terephthalic acid and a glycol mixture comprising a 9:1 weight ratio of 2,2-bis[4-(2-hydroxyethoxy)phenyl] propane and ethylene glycol (binder) and 0.0065 g. of the spectral sensitizing dye indicated by identifying number from above Table A. Each solution is coated on an aluminum surface maintained at 25 C., and dried. All operations are carried out in a darkened room. A sample of each coating is uniformly charged by means of a corona to a potential of about 600 volts and exposed through a transparent member bearing a pattern of varying optical density to a 3000 K. tungsten source. The resultant electrostatic image pattern is then rendered visible by cascading a developer composition comprising finely divided colored thermoplastic electrostatically responsive toner particles carried on glass beads over the surface of the element. The image is then developed by deposition of the toner in an imagewise manner on the element. (Other development techniques such as those described in US. 2,786,439; 2,786,440; 2,786,441; 2,811,465; 2,874,063; 2,984,163; 3,040,704; 3,117,884; Re. 25,779; 2,297,691; 2,551,582; and in RCA Review, vol. 15 (1954), pp. 469-484, can be used with similar results.) An image is formed on each sample, as indicated in Table I. Another sample of each coating is tested to determine its electrical speed and maximum sensitivity pea-k. This is accomplished by giving each element a positive or negative charge (as indicated in Table I) with a corona source until the surface potential, as measured by an electrometer probe, reaches 600 volts. It is then exposed to light from a 3000" K. tungsten source of 20-foot candles at the exposure surface. The exposure is made through a stepped density gray scale. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potention, V0, to some lower potential, V, whose exact value depends on the actual amount of exposure in meter-candle-seconds received by the area. The results of these measurements are plotted on a graph of surface potential V vs. log exposure for each step. The actual speed of each element is expressed in terms of the reciprocal of the exposure required to reduce the surface potential by volts. Hence, the speeds given in Table I are the numerical expression of 10 divided by the exposure in metercandle-seconds required to reduce the 600 volts charged surface potential by 100 volts. The results are shown in Table I below.

Referring to the above Table I, it will be seen that the control example containing the same photoconductor but no dye shows speeds of only 8 and 7 for the positively and negatively charged surfaces, respectively, whereas the corresponding values for those of the invention represented by Examples 1 to 14 are clearly of a different order of magnitude. For example, the highest speed is shown by Example 2 (Dye No. V.) of 1400 and 720 for the positively and negatively charged surfaces, respectively, with a maximum sensitivity peak 540 nm., thus indicating a speed increase over that of the control by a factor of about 175 for the positively charged and about 103 for the negatively charged. Also of great significance is the extension of the absolute sensitivity in some cases to the region of 550 nm. Even with the least speed shown for the compositions and elements of the invention as illustrated by Example 12 (Dye No. XXIII) the improvement in speed is impressive in comparison with that of the control by factors of about 3 and 4 for the positive charged and negatively charged surfaces, respectively. Results similar to those in Examples 1-14 are obtained when the dyes used therein are replaced with like amounts of dyes I, III, IV, VII, VIII, XII, XIV-XVI, XXI, XXII, XXIV, or any of the dyes mentioned after Table A.

Similar results to those shown in above Table I are obtained when, for example, the organic photoconductor 4,4'-bis(diethylamino)-2,2' dimethyltriphenylmethane is replaced with 0.15 g. of triphenylamine (using the p toluene sulfonate salt of each dye), or 1,3,5-triphenyl-2- pyrazoline, or 2,3,4,5-tetraphenylpyrrole, or 4,4'-bisdiethylaminobenzophenone or when other dyes of the invention embraced by Formula I above are used. These results show that the dyes of this invention effectively spectrally sensitize a wide variety of organic photoconductors. The dyes of this invention are not in themselves photoconductive. Also it should be noted that the above mentioned photoconductors when used alone have very low photoconductive speed to visible light. However, as shown by the tests, the combination of the dyes of the invention with the photoconductors of the invention provide compositions and elements of outstanding speed and excellent quality of image.

This invention is highly unexpected because dyes previously suggested for spectral sensitizers impart weak spectral sensitization to organic photoconductors. Typical dyes proposed by the prior art as spectral sensitizers, which produce weak spectral sensitization in these systems, are shown in Table D below.

' TABLE D Dye Identification:

Name

A Pinacyanol.

B Kryptocyanine.

C Anhydro-3-ethyl-9-methyl 3 (3-sulfo butyl)-thiacarbocyanin e hydroxide.

D 3,3'-diethyl 9 methylthiacarbocyanine bromide.

E 3-carboxylmethyl-5-[(3 methyl 2 thiazolidinylidene) 1 methylethylidene] rhodanine.

F Anhydro 5,5 dichloro 3,9-diethyl-3'- (3 sulfobutyl)thiacarbocyanine hydroxide.

G.. 1-ethyl-3-methylthia-2-cyanine chloride.

H 1,1diethyl-2,2-cyanine chloride.

The dyes of this invention desensitize conventional negative type photographic silver halide emulsions.

Although the invention has been described in considera- 'ble detail with particular reference to certain preferred embodiments thereof, it will be understood that 'variations and modifications can be effected within the spirit and scope of the invention as described hereinabov'e, and as defined in the appended claims.

We claim:

1. A composition of matter comprising an organic photoconductor spectrally sensitized with a methine dye comprising first and'second 5- to 6-memberecl nitrogen containing heterocyclic nuclei joined by a dimethine linkage; the first of said nuclei comprising an irnidazole ring joined at the S-carbon atom thereof to said linkage, said imidazole ring having fused, on the [a] side thereof, the non-metallic atoms necessary to complete at least one ring containing from 5 to 6 atoms; and, the second of said nuclei being an electron-accepting nucleus of the type used in cyanine dyes joined at a carbon atom thereof to said linkage, to complete said dye.

2. A composition as defined in claim 1 wherein sai first nucleus of said dye is selected from the group consisting of an imidazo[1,2-a] pyridine nucleus; an imidazo [2,l-b]thiadole nucleus; an imidazo [2,1-b] oxazole nucleus; an imidazo [2,l-b]-selenazole nucleus; an imidazo [2,l-b] 1,3,4-thiadiazole nucleus; and, an imidazo[l,2-b] pyridazine nucleus.

3. A composition as defined by claim 1 wherein said electron-accepting nucleus of said dye is substituted by a nitro group on a carbon atom thereof.

4. A composition as defined by claim 1 wherein said electron-accepting nucleus of said dye is an imidazo [4,5-b] -quinoxaline nucleus.

5. A composition as defined by claim 1 wherein said organic photoconductor is selected from the group consisting of: triphenylamine; a l,3,5-triphenyl-2-pyrazoline; a 4,4'-bis-(dialkylamino)-2,2'-dialkyltriarylamine; a 2,3,4, S-tetraarylpyrrole; and a 4,4 -bis-dialkylaminobenzophenone.

6. A composition as defined by claim 1 wherein said organic photoconductor and said dye are incorporated in an insulating binder.

7. A composition of matter comprising an organic photoconductor spectrally sensitized with a methine dye selected from those having one of the following general formulas:

wherein n represents a positive integer of from 1 to 2; L represents a methine linkage; R represents a member selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group; R and R each represents a member selected from the group consisting of an alkyl group, an alkenyl group and an aryl group; Z represents the non-metallic atoms necessary to complete a desensitizing nucleus containing from 5 to 6 atoms; X represents an acid anion; and Q represents the non-metallic atoms required to complete a fused heterocyclic ring containing from 5 to 6 atoms.

8. A composition as defined in claim 7 wherein said Q of said dye represents the non-metallic atoms necessary to complete a nucleus selected from the group consisting of an imidazo[1,2-a]pyridine nucleus; an irnidazo[2,lbJthiazole nucleus; an imidazo[2,l-b]-1,3,4-thiadiazo1e nucleus; and an imidazo[l,2-b]-pyridazine nucleus.

9. A composition as defined in claim 7 wherein said Z of said dye represents the non-metallic atoms necessary to complete a nucleus selected from the group consisting of a nitrobenzothiazole nucleus; a nitrobenzoxazole nucleus; at nitrobenzoselenazole nucleus; a 3,3-dialkyl-5- nitro-3H-indole nucleus; and an imidazo[4,5-b]quinoxaline nucleus. 1

17 10. A composition as defined by claim 7 wherein said organic photoconductor has the following formula:

wherein each of D, E and G is an aryl group and J is selected from the group consisting of a hydrogen atom, an

alkyl group and an aryl group, at least one of D, E and G containing an amino substituent selected from the group consisting of a secondary amino group and a tertiary amino group.

11. A composition as defined by claim 7 wherein said organic photoconductor is selected from the group consisting of: triphenylamine; l,3,S-triphenyl-Z-pyrazoline; 4,4 bis(diethylamino) 2,2 dimethyltriphenylamine; 2,3,4,S-tetraphenylpyrrole; and, 4,4 bis diethylaminobenzophenone.

12. A composition as defined by claim 7 which comprises from 1 to 75 parts by weight of said photoconductor, said photoconductor being spectrally sensitized with from .01 to 10 parts by weight of said dye.

13. A composition of matter comprising from 1 to 75 parts by weight of an organic photoconductor selected from the gorup consisting of: triphenylamine; 1,3,5-triphenyl-2-pyrazoline; 4,4'-bis-diethylamino-2,2-dimethyltriphenylmethane; 2,3,4,5-tetraphenylpyrrole; 4,4-bis-diethylaminobenzophenone; said organic photoconductor being spectrally sensitized with from .01 to 10 parts by weight of a dye selected from the group consisting of:

a 1,3-diphenyl-2- [2- (2-phenylimidazo 1,2-a] -pyrid-3 -yl) vinyl] imidazo [4,5 -b] quinoxalinium salt;

a 3-ethyl-2-{2- [8-methyl-2- (4nitrophenyl) imidazo[ 1,2-a]

pyrid-3 -yl]vinyl}-6-nitrobenzothiazolium salt;

a 1,3 diethyl 2-{2-[8-methyl-2-(4nitrophenyl)imidazo 1,2 a]pyrid-3-yl]-vinyl}imidazo [4,5-b] quinoxalinium salt;

a 1,3,3 trimethyl-2-{2-[8-methyl-2-(4-phenylazophinyl) imidazo[1,2-a]pyrid-3-yl]-vinyl}-5-nitro-3H indolium salt;

a 3 ethyl 2{2-[8-methyl-2-(4-phenylazophenyl)imidazo 1,2-a] pyrid-3-yl] vinyl}-6-nitrobenzothiazolium salt;

a 1,3 diethyl 2-{2-[8-methyl-2-(4-phenylazophnyl)imidazo 1,2-a] pyrid -3-yl] vinyl}imidazo [4,5 -b] quinoxalinium salt;

a 6-ch1oro-2-{2-[7-methyl-2-(4-nitrophenyl)imidazo[1,2-

a]pyrid 3 vyl]vinyl}-1,3-diphenylimidazo[4,5-b1quinoxalinum salt;

a 6 -chloro-2-{2-[6-(4-nitrophenylmidiza[2,1-b1thiazol-5- yl]vinyl} 1,3 diphenylirnidazo[4,S-b]quinoxalinium salt;

:1 1,3,3 trimethyl-2-{2-methyl-6-(4-nitrophenyl)imidazo [2, l -b 1,3,4-thiadiazol-5-yl] vinyl}-5-nitro-3H-indolium salt;

a 3 ethyl-2-{2-[2-methyl-6-(4-nitrophenyl)imidazo[2,l-

b]-1,3,4 thiadiazol-S-yl]vinyl}-6-nitrobenzothiazolium salt;

a 1,3 diethyl 2-{2-[2-methyl-6-(4-nitrophenyl)imidazo [2,1-b] 1 3-4-tliiadiazol-5-yl]vinyl}imidazo[4,5-b] quinoxalinium salt;

a 2 {2-[2 (4-bromophenyl)-6-methoxyimidazo[1,2-b] pyridazin 3 yl]vinyl}-1,3,3-trimethyl-5-nitro-3H-indolium salt;

a 2 {2-[2-(4-bromophnyl)-6-methoxyimidazo[1,2-b]pyridazin 3 yl]vinyl}-3-ethyl-6-nitrobenzothiazolium salt; and

a 2-{ 2- [2- (4-bromophenyl) -6-methoxyimidazo 1,2-b] -pyridazin 3-yl]vinyl}-l,3-diethylimidazo [4,5-b]quinoxalinium salt.

14. An electrophotographic element comprising a conductive support having thereon a layer comprising an organic photoconductor in an insulating binder, said "alkyl group, an alkenyl group and an aryl group; X6

organic photoconductor being spectrally sensitized with a methine dye comprising first and second 5- to 6-member ed nitrogen containing heterocyclic nuclei joined by a dimethine linkage; the first of said nuclei comprising an imidazole ring joined at the S-carbon atom thereof to said linkage, said imidazole ring having fused, on the [a] side thereof, the non-metallic atoms necessary to complete at least one ring containing from 5 to 6 atoms; and, the second of said nuclei being an electron-accepting nucleus of the type used in cyanine dyes joined at a carbon atom thereof to said linkage, to complete said dye.

, 15'. An electrophotographic element as defined in claim 14 wherein said first nucleus is selected from the group consisting of an imidazo[1,2-a]pyridine nucleus; an imidazo [2,l-b] thiazole nucleus; an imidazo 2, l-b] oxazole nucleus; an imidazo[2,l-b1selenazole nucleus; an imidazo- [2,l-b]-1,3,4-thiadiazole nucleus; and, an imidazo[1,2-b] I pyridazine nucleus.

16. An electrophotographic element as defined in claim 14 'wherein said electron-accepting nucleus of said dye issubstituted by a nitro group on a carbon atom thereof.

17. An electrophotographic element as defined in claim 14"wherein said electron-accepting nucleus of said dye is an imidazo[4,5-b]quinoxaline nucleus.

18. An electrophotographic element as defined in claim 14 wherein said organic photoconductor is selected from theLgroup consisting of: a triarylamine'; a 1,3,5-triphenyl- 2-pyraz0line; a 4,4 bis (dialkylamino)-2,2-dialkyltriarylamine; a 2,3,4,5-tetraarylpyrr0le; and a 4,4'-bis-dialkylaminobenzophenone.

19. An electrophotographic element comprising a conduct ive support having thereon a layer comprising an organic photoconductor spectrally sensitized with a dye selected from those represented by the following formula:

wherein n represents a positive integer of from 1 to 2; L represents a methine linkage; R represents a member selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group; R and R each reprevsents a'membcr selected from the group consisting of an represents an acid anion; Z represents the non-metallic atoms necessary to complete a desensitizing nucleus containing 5 to 6 atoms; and Q represents the non-metallic atoms required to complete a fused heterocyclic ring containing from 5 to 6 atoms.

20. An electrophotographic element as defined in claim 19 wherein said Q of said dye represents the non-metallic atoms necessary to complete a nucleus selected from the group consisting of an imidazo[1,2-a]pyridine nucleus; an imidazo[2,1-b]thiazole nucleus; an imidazo[2,l-b1- 1,3,4-thiadiazole nucleus; and an imidazo[1,2-b]pyridazine nucleus.

21. An electrophotographic element as defined in claim 20 wherein said Z of said dye represents the non-metallic atoms necessary to complete a nucleus selected from the group consisting of a nitrobenzothiazole nucleus; a nitrobCIlZdXfiZOlC nucleus; a nitrobenzoselenazole nucleus; a 3,3-dialkyl-5-nitro-3H-indole nucleus; and an imidazo[4,5- b]quinoxaline nucleus.

22. An electrophotographic element as defined in claim 21 wherein said organic photoconductor has the following formula:

wherein each of D, E and G is an 'aryl group and] is selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group, at least oneof D, E and 6 containing an amino substituent selected from the group consisting of a secondary amino group and a tertiary amino group. p

23. An electrophotographic element as defined in claim 21 wherein said organic photoconductor is selected from 25. Ad electrophotographic element as defined in claim 24 wherein said organic photoconductor and said dye are incorporated in an insulating binder.

26. An electrophotographic element as defined in claim 25 wherein said organic photoconductor and said dye are dispersed in from 25 to 99 parts by weight of a polyester of terephthalic acid and a glycol mixture consisting of a 9:1 weight ratio of 2,2 bis [4 (2 --hydroxyethoxy)- phenyl]-propane and ethylene glycol as insulating binder.

References Cited UNITED STATES PATENTS 3,132,942 5/1964 Stewart 961 3,143,544 8/1964 Van Dormael 260240 3,326,688 6/1967 Jenkins et a1. 96102 3,455,684 7/1969 ,Depoorter et al. 96l.7 3,468,661 9( 1969 Libeer et al. 96l.7X

2O .GEOR GE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. 01. X.R. 

